A. Field of the Invention
The present invention relates to a piezoelectric actuator, and more particularly, to a piezoelectric actuator that includes driver electrodes formed on two opposing surfaces of a piezoelectric plate such that in response to an applied voltage, the piezoelectric plate is deformed into a curved or arcuate shape.
B. Description of the Related Art
In positioning of electronic microscopes, position control of a head in magnetic recording or optical recording, or in any electronic equipment that involves controlling movements or motion of an object, small sized actuators are required to rapidly control minute displacement.
As such actuators, generally, magnetic actuators have been used which combine windings and magnetic substances to generate and control displacement. Those magnetic actuators, however, require a large numbers of windings and a have complicated structures, suffering from a latent problem of being difficult to integrate into small devices. In addition, magnetic actuators often require a large current in high-speed operation due to a high inductive response in their components.
In place of magnetic actuator, piezoelectric actuators are sometimes used which have piezoelectric materials that are configured to undergo deformation from a generally planar or rectangular shape to an arcuate or curved shape in response to an applied voltage. The piezoelectric actuators are provided with electrodes in order to apply a voltage across opposing surfaces of a piezoelectric plate. The piezoelectric plate is made of two layers of semi-conducting piezoelectric porcelain or such piezoelectric ceramic as ferroelectric crystal, such that displacement occurs in one layer in a first direction, and in another layer in an opposite direction causing curvature in response to the applied voltage. In order to simplify the following descriptions of the related art and of the present invention, the above described displacement, where a rectangular member deforms to have a curved shape, will hereinafter be referred to as curving displacement.
The above piezoelectric actuators undergo curving displacement due to the transverse piezoelectric effects of piezoelectric materials employed in the two layers. The thin layers can be thin-sheet plates of piezoelectric material that are fixed to one another. Electrodes are formed on two opposing surfaces thereof. To obtain a large amount of displacement for an applied voltage, such thin-sheet plates may be provided in a laminated structure.
Such a lamination-structured piezoelectric actuator suffers from a problem in that only a small amount of displacement is realized as a result of an applied voltage. Typically, the applied voltage must be very large in many cases, requiring, for example, several hundreds of volts of applied voltage to provide several micrometers of displacement. In addition, the laminated structure is a complicated construction, which may result in a hysteresis effect in the displacement of joined portions of the piezoelectric plate.
There are also known bimorph type piezoelectric actuators, in which two piezoelectric plates are joined together to generate curving displacement in response to application of an electric field. The electric field causes the plate to expand in mutually opposite directions. In such bimorph type piezoelectric actuators, large amounts of displacement are expected for a given voltage. However, there is typically a hysteresis effect associated with the displacement at the joined portion between the two piezoelectric plates. In addition, in order to give large amounts of displacement in response to an applied voltage, the bimorph type piezoelectric actuators require piezoelectric plates that have a reduced thickness. The reduced thickness in turn give rise to problems associated with natural resonance frequency in the direction of curving displacement, in particular, undesirable resonance occurs in the form of pulse responses just after the electric field is applied and just after the electric field is removed. With this, the service frequency must be distant in value from the resonance frequency, giving rise to a problem of restrictions in service conditions. Also, when the piezoelectric plates are provided with viscosity to suppress the resonance phenomenon, the response characteristics may deteriorate.
The above-mentioned bimorph type piezoelectric actuators may suffer from a hysteresis effect after the electric field is applied such that the amount of displacement at the joined portions between the piezoelectric plates changes over an extended period of time. Further, at the same time, actuators made with minute ferroelectric crystal and other piezoelectric ceramic may suffer from domain state changes with changing voltages, so that there may be some hysteresis of the displacement vs. voltage characteristics latent in the piezoelectric plates themselves. Also, creep phenomenon, in which the displacement amount would change gradually even with a constant application voltage, has been found to be influenced by the manner in which the piezoelectric plates are joined.
By using reverse-polarized plates made of ferroelectric monocrystal such as lithium niobate (LiNbO3) or lithium tantalate with inverted polarization directions near the center of the plate thickness, it is only possible to configure a bimorph type piezoelectric actuator by forming electrodes on each opposing surface. Such reverse-polarized plates made of ferroelectric monocrystal are able to obtain a large amount of displacement for a given applied voltage because of a large corresponding electromechanical coupling factor. Further, in such a device suppression of hysteresis and creep phenomenon is possible because there are no joined portions. They may have, however, resonance vibration having a large Q value in the direction of curving displacement. If such piezoelectric plates are used in an actuator structures,.undesirable resonance vibration may develop with respect to time-related changing drive voltage, giving rise undesirable resonance characteristics immediately after voltage changes, in particular, pulse response.
One solution to eliminate such an undesirable resonance phenomenon may be to provide sensor electrodes that detect amounts of displacement of the piezoelectric plates. The sensor electrodes should be separate from driver electrodes so that an output signal from the sensor electrodes may be fed back to a driving signal inputted to the driver electrodes. In the bimorph type piezoelectric actuators, it is possible to provide the main driver electrode on a. first surface and a ground electrode on a second surface that is opposite from the first surface, and sensor electrodes on the first surface separate from the main driver electrode.
Piezoelectric actuators utilizing piezoelectric plates with a thin-sheet plate configuration are known to have a natural resonance frequency in the curving displacement direction. Therefore, undesirable resonance phenomenon that may occur immediately after changes in pulse voltages is expectedly eliminated by feeding back an output signal from the above-mentioned sensor electrodes to the driving signal.
Those piezoelectric actuators, however, may sometimes suffer from a resonance phenomenon due to a secondary vibration mode in a higher harmonic different from that of a primary vibration mode that is caused by a primary resonance frequency of the piezoelectric plates. The secondary vibration mode is believed to be due to changes in the charge generation distributions that are caused by the asymmetry of the electrode geometry or the inclusion of the sensor electrodes themselves.
Therefore, if sensor electrodes are provided to suppress the primary vibration mode in the primary resonance frequency of the piezoelectric plates, the geometry of the sensor electrodes may have an influence on the geometry of the main driver electrode, giving rise to a resonance phenomenon due to a secondary vibration mode. The phenomenon is particularly highly expected when reverse-polarized plates are used which are made of ferroelectric monocrystal with a high Q value such as lithium niobate.
It is therefore an object of the present invention to provide a piezoelectric actuator with means to suppress the resonance phenomenon in the response characteristics of the actuator. In particular, it is desirable to suppress the secondary vibration mode corresponding to a second resonance frequency different from a first resonance frequency, in the direction of curving displacement.
In accordance with one aspect of the present invention, a piezoelectric actuator includes a piezoelectric element configured to undergo curving displacement in response to an applied driving signal. There are driver electrodes formed on opposing surfaces of the piezoelectric element, the driving signal being applied to the driver electrodes. Further, the piezoelectric actuator and electrodes are configured to eliminate electromechanical coupling that causes a primary vibration that is a result of a primary resonance frequency of the piezoelectric element. Further, the piezoelectric element and electrodes are configured to eliminate electromechanical coupling which causes a secondary vibration during curving displacement of the piezoelectric element as a result of a secondary resonance frequency, the primary and secondary resonance frequencies being different from one another.
Accordingly, the secondary vibration mode can be suppressed to improve the response characteristics of the piezoelectric actuator.
Preferably, the piezoelectric element includes two piezoelectric plates laminated together, the piezoelectric plates being oriented with reverse polarization with respect to one another.
Preferably, the piezoelectric plates of the piezoelectric element are each bimorph type piezoelectric plates.
Preferably, each of the piezoelectric plates are made of lithium niobate monocrystal that have a polarized orientation.
Preferably, the electrodes include a main driver electrode formed on a first surface of the opposing surfaces and a ground electrode provided in a second surface. of the opposing surfaces.
Preferably, the piezoelectric element is formed with a fixture portion at one end thereof in a longitudinal direction, the fixture portion being fixed to a support member.
Preferably, the main driver electrode extends to the fixture portion.
Preferably, the piezoelectric actuator further includes adjustment electrodes connected to the main driver electrode, the adjustment electrodes being formed in proximate the main driver electrode.
Preferably, the adjustment electrodes include a plurality of auxiliary driver electrodes configured for selective connection to the main driver electrode.
Preferably, the electrodes further include a sensor electrode configured to detect displacement and acceleration of the piezoelectric element in a direction corresponding to the curving displacement. The sensor electrode is formed on the first surface proximate the main driver electrode.
Preferably, the sensor electrode is formed on a central portion of the first surface of the piezoelectric element. The main driver electrode extends at least partially around the sensor electrode, and the sensor electrode and the main driver electrode are symmetrical with respect to a longitudinal centerline of the piezoelectric element.
Preferably, a second ground electrode is formed between the sensor electrode and the main driver electrode, the second ground electrode extending at least partially around the sensor electrode.
Preferably, a feedback circuit is connected to the electrodes, the feedback circuit being configured to suppress resonance in the primary vibration mode by providing a feedback signal from the sensor electrode to a driving signal inputted to the driver electrode.
Preferably, the feedback circuit includes a bandpass filter set for a frequency proximate the primary resonance frequency. The bandpass filter is inserted into the feedback circuit.